In conventionally fabricating a semiconductor integrated circuit in which a generally flat semiconductor chip (or body) is housed in a suitable package, a thin pattern of an electrically conductive material connecting to the electrical leads of the package is formed on the chip to provide appropriate connection to the semiconductive elements in the chip. The conductive pattern is created by depositing a thin film (or layer) of the material on the chip and then removing the undesired portion of the film. The material is normally aluminum but may include a small percentage of copper and/or silicon with the Al. The weight percentage of Cu, if included, is no more than 4%. The same holds for Si.
One of the principal techniques for applying the thin conductive film on the body is evaporative deposition. In this technique, the chip is placed in a vacuum chamber containing a high-purity source of aluminum. If Cu and/or Si is also used, it may be supplied either from the Al source or separately. An electron beam is directed toward the source so as to cause part of it to vaporize. After opening a shutter between the source and body, atoms of the resulting vapor evaporate towards the chip where they accumulate to form the thin film. The chip is typically mounted on a planetary apparatus that rotates so as to improve the film coverage on the body which, although macroscopically flat, microscopically has numerous hills and valleys.
FIGS. 1a-1f generally illustrate the evaporative deposition process from a microscopic viewpoint in which only aluminum is deposited. A source 10 which is effectively a point-source insofar as the chip is concerned provides the Al. The evaporated Al atoms move in a direction 12 towards a surface 14 of the body. Macroscopically, surface 14 is substantially planar. A perpendicular to surface 14 as viewed macroscopically, or to those portions of surface 14 as viewed microscopically that are not inclined with respect to the general plane of the chip, is at an impingement angle .theta. to direction 12.
Impingement angle .theta. normally reaches a highest maximum value in one particular direction and a lowest maximum value in the opposite direction. These maxima are both represented as .theta..sub.MAX in FIGS. 1a-1f. In rotating three-dimensionally in the planetary, the chip then swings back and forth from about .theta..sub.MAX in one direction as represented by the solid-line versions of surface 14 in FIGS. 1a-1f to about .theta..sub.MAX in the opposite direction as indicated by the dashed-line versions of surface 14. The two values of .theta..sub.MAX depend on the location of the chip on the planetary. If it is optimized, .theta..sub.MAX is substantially constant at any of its chip locations at a value typically around 30.degree..
The slopes of the hills and valleys in surface 14 are roughly represented by inclined surface portons 16 (of which only one is shown in each of FIGS. 1b-1f) that each have a topographical surface angle .mu. relative to the general plane of the body--i.e., to the non-inclined portions of surface 14. Surface angle .mu. may be positive or negative, a positive .mu. value representing a valley while a negative .mu. value represents a hill. For convenience, negative .mu. values are not illustrated since hills are schematically equivalent to valleys here.
Shadowing occurs when atoms from source 10 cannot directly impinge on inclined portion 16 because a portion of the body along surface 14 is in the way. Depending on the value of .mu., inclined portion 16 may never be shadowed during a complete planetary rotation of the body, may be shadowed during part of the planetary rotation, or may be shadowed during the entire rotation. When inclined portion 16 is shadowed, the shadowed area also includes the underlying portion of surface 14 that portion 16 itself shadows. FIG. 1a represents the zero-point situation in which no shadowing occurs at all because the illustrated portion of surface 14 is microscopically flat. FIG. 1b indicates the case where .mu. is positive but less than 90.degree.-.theta..sub.MAX so that portion 16 is never shadowed. FIG. 1c represents the onset of shadowing where .mu. equals 90.degree.-.theta..sub.MAX as shown by the solid-line version of surface 14. FIG. 1d indicates the situation where .mu. lies between 90.degree.-.mu..sub.MAX and 90.degree.+.theta..sub.MAX ; as shown by the solid-line version of surface 14, portion 16 is shadowed during part of the planetary rotation. FIG. 1e represents the onset of complete shadowing where .mu. equals 90.degree.+.theta..sub.MAX as shown by both versions of surface 14. Finally, FIG. 1f indicates the case where .mu. is greater than 90.degree.+.theta..sub.MAX ; portion 16 is shadowed during the entire planetary rotation.
Generally, surface 14 is formed with .vertline..mu..vertline. not exceeding 90.degree.+.theta..sub.MAX anywhere so that no part of the body is completely shadowed during Al deposition. The cases shown in FIGS. 1a-1d up to FIG. 1e thus represent the overall situation of primary interest.
The vapor deposition rate R at which Al atoms from source 10 accumulate on surface 14 is conventionally constant at around 15 angstroms/second on the non-inclined portions of surface 14. FIG. 2 illustrates the duty cycle for such a prior art deposition process. In this process, the deposition-controlling shutter is opened shortly after source 10 is initially activated. This enables Al atoms from source 10 to impinge on surface 14 as the R rate rises rapidly to 15 angstroms/seconds while the body rotates in the planetary apparatus. When about 18,000 angstroms of Al have accumulated on surface 14, the shutter is closed to return the R rate to zero.
The 15-angstroms/second rate is moderately high, so chosen as to provide maximum throughput with good Al coverage. As long as there is substantially no shadowing --i.e., .vertline..mu..vertline. is less than 90.degree.-.theta..sub.MAX everywhere along surface 14, the resultant Al film is normally continuous and does not have serious continuity defects. However, when there is partial shadowing i.e., .vertline..mu..vertline. lies between 90.degree.-.theta..sub.MAX and 90.degree.+.theta..sub.MAX for some surface portions 16, continuity defects appear that seriously damage the integrity of the film. These defects include "thinning" in which the aluminum layer is so thin that cross-sectional current densities exceed design limits and "microcracking" in which cracks occur in the film due to insufficient coalescense of the Al grains. Another of these defects is "tunneling" in which gaps occur next to or under portions 16, especially those ones that are highly inclined. These defects need to be avoided.